The present invention relates to a process for preparing a silicon carbide powder having a purity sufficient for its use in semiconductor device-manufacturing equipment (hereinafter merely referred to as "semiconductor equipment"). Semiconductor equipment is used to carry silicon wafers or similar substrates and shield them from air in the manufacture of semiconductor devices such as integrated circuits. Semiconductor equipment usually includes one or more wafer carriers (called boats) and tubes and other parts. The manufacture of semiconductor devices usually involves one or more heat treatment steps at high temperatures such as oxidation, CVD, PVD, SOI (silicon on insulator), or thermal diffusion of an impurity as a dopant, so semiconductor equipment is required to resist such high-temperature atmospheres.
Semiconductor equipment which has conventionally been used is usually made of quartz glass (fused silica) or silicon. Semiconductor equipment made of quartz glass has a quite limited service life under the conditions existing during heat treatment at a high temperature since it is susceptible to deformation or distortion during heat treatment due to the relatively low softening point of quartz glass which is in the vicinity of 1100.degree. C. Furthermore, when kept at a high temperature for a prolonged period, it may become devitrified and broken due to phase transition into alpha-cristobalite.
Semiconductor equipment made of silicon has the problem that it is contaminated with the impurities used as dopants in thermal diffusion steps by undesirable diffusion of the impurities into the equipment. The impurities build up in the equipment during use, thereby causing contamination of wafers which are being processed therein.
Silicon carbide (SiC) is chemically stable and resistant to corrosion at high temperatures and its strength and stiffness are much higher than those of quartz glass. In view of these properties, semiconductor equipment made of sintered silicon carbide is sometimes used. However, such semiconductor equipment is rarely used in processing of silicon wafers of high quality which are sensitive to metallic impurities, since presently available silicon carbide contains a considerable amount of metallic impurities, which are often evaporated to generate a gas during thermal diffusion treatment, resulting in contamination of wafers.
Accordingly, there is a need for pure silicon carbide powder which is substantially free from metallic impurities, i.e., with a content of 1 ppm or less for each metallic impurity, as a raw material for sintered silicon carbide.
Silicon carbide has two crystal forms, alpha-form (hexagonal) and beta-form (cubic), of which beta-silicon carbide in powder form, which has a structure similar to diamond, is more suitable for use in the manufacture of semiconductor equipment. Known methods used to prepare beta-silicon carbide powder involve (1) a reaction of SiO.sub.2 with C, (2) a reaction of metallic Si with C, or (3) a vapor phase reaction of an Si compound, e.g., SiCl.sub.4, with a hydrocarbon. Method (1) is used in commercial production of beta-silicon carbide powder since the starting materials are inexpensive and the reaction can be easily controlled.
The preparation of beta-silicon carbide powder by a reaction of SiO.sub.2 with C proceeds through either the following
______________________________________ reaction scheme (a) or (b) at a high temperature: ______________________________________ Reaction Scheme (a): SiO.sub.2 + 3C .fwdarw. SiC + 2CO(g) Reaction Scheme (b): SiO.sub.2 + C .fwdarw. SiO(g) + CO(g) SiO(g) + 2C .fwdarw. SiC + CO(g) ______________________________________
where (g) means that the material is in a gaseous phase.
Of these reaction schemes, scheme (a) is usually employed because scheme (b) includes a heterogeneous solid-vapor reaction, thereby making it difficult to form a uniform powder product having even particle diameters, and the product of reaction scheme (b) is contaminated with a small amount of alpha-silicon carbide.
The most popular process for the preparation of beta-silicon carbide powder using the above-described reaction scheme (a) is the Acheson process. The Acheson process comprises reacting a siliceous material (SiO.sub.2 or its precursor) and a carbonaceous material (C or its precursor) by heating a mixture of these two solid materials in powdery form in a batchwise electric-resistance furnace known as an Acheson-type furnace to produce silicon carbide in lumps. The Acheson process has the drawbacks of poor operating efficiency and a deteriorated work environment since it requires the removal of a side wall of the furnace for the recovery of the product in each reaction cycle. Moreover, each of the solid starting materials contains an appreciable amount of impurities, and since the resulting silicon carbide product in lumps must be pulverized into powder, additional metallic impurities are incorporated into the powder product during pulverization of the hard, lump silicon carbide. Therefore, the Acheson process has another significant drawback that the product is inevitably contaminated with a considerable amount of impurities.
In order to improve the operating efficiency, it has been proposed in Japanese Patent Publication No. 58-18325(1983) and No. 58-34405(1983) that a mixture of the powdery starting materials be shaped by use of a binder such as pitch, thereby making it possible to directly produce a beta-silicon carbide powder without a pulverization step. Japanese Patent Application Kokai No. 61-6110(1986) discloses an improved continuous process for the preparation of beta-silicon carbide powder which comprises preparing a starting mixture consisting of a solid siliceous material, a solid carbonaceous material, a liquid silicon compound, and a curable organic compound having polymerizable or cross-linkable functional groups, preheating the starting mixture so as to cure the organic compound and solidify the mixture, and calcining the solidified mixture in a non-oxidizing atmosphere.
It is also known that a starting mixture is homogenized using a liquid starting material in order to produce a silicon carbide powder having a uniform particle diameter or shape. For example, it is proposed in Japanese Patent Application Kokai No. 57-88019(1982) to prepare a starting mixture by treating a carbonaceous material with a silicic acid solution and calcining the mixture in a non-oxidizing atmosphere. Preferably, the carbonaceous material is also a liquid substance and the mixture is prepared in a liquid state. A small amount of silica sol is undesirably formed in the starting mixture and adversely affects the quality of the product. In order to eliminate this drawback, it is disclosed in Japanese Patent Publication No. 1-42886(1989) to use a mixture comprising a liquid siliceous material, a curable or polymerizable liquid organic compound capable of forming carbon upon heating, and a polymerization or curing catalyst which is compatible with the liquid organic compound to form a homogeneous solution. The mixture is reacted to form a cured body containing Si, O, and C as an SiC precursor, which is then calcined in a non-oxidizing atmosphere to give a beta-silicon carbide powder.
However, in the above-described prior-art processes for preparation of beta-silicon carbide powder, it was difficult or impossible to produce a beta-silicon carbide powder of high purity having a content of 1 ppm or less for each metallic impurity, a purity level which is accepted in the manufacture of semiconductor equipment. It has been found that the final products obtained in these processes are contaminated with metallic impurities in considerable amounts, e.g., on the order of 3 ppm or more for one or more impurity metals, and this level of contamination is not acceptable in semiconductor equipment. Although washing is an effective means for removing impurities, it is quite difficult in a commercial process to decrease an impurity level to 1 ppm or less by washing.